It has been known that, among prostaglandin derivatives, there are many derivatives that reduce ocular tension when they are topically applied (EP 0170258 and EP 0253094), and it has been reported that, among 13,14-dihydro-17-phenyl-18,19,20-trinor-PGF2α-isopropyl esters, an optically active substance represented by the following formula (hereinafter, this compound is referred to as “latanoprost”) has high efficacy and can be used as a therapeutic agent for glaucoma (WO90/02553).

Since latanoprost has a chemical structure similar to that of prostaglandin F2α, a preparation method based on the Corey method (J. Am. Chem. Soc., 91(20), pp. 5675-5677, 1969) has been proposed for the synthesis thereof (method of preparing latanoprost from so-called Corey lactone). The key steps for preparing latanoprost by this method are (1) protection of hydroxyl groups at the positions corresponding to the 11th and 15th positions of latanoprost, (2) introduction of a specific configuration ((R)-configuration) of the hydroxyl group at the position corresponding the 15th position of latanoprost, and (3) introduction of a single bond linking 13th and 14th positions of latanoprost.
In relation to these steps, WO92/02496 discloses a method of protecting hydroxyl groups at the positions corresponding to the 11th and 15th positions of latanoprost, and then hydrogenating the double bonds to form a single bond linking 13th and 14th positions of latanoprost.
WO93/00329 discloses a method of introducing a carboxyalkenyl group at the position corresponding to the 8th position of latanoprost as a side chain (skeleton of the 1st to 7th positions) without protecting the hydroxyl groups at the positions corresponding to the 11th and 15th of latanoprost.
WO03/08368 discloses a method of using a boron asymmetric reducing reagent and (−)-chlorodiisopinocamphenylborane to introduce hydroxyl group of a specific configuration at the 15th position of latanoprost and utilizing a ring-opened lactone as a preparation intermediate.
WO01/55101 discloses a method of introducing a carboxyalkenyl group at a position corresponding to the 8th position of latanoprost as a side chain (skeleton of the 1st to 7th positions) without protecting the hydroxyl group at the position corresponding to the 11th position of latanoprost.
US 2006/0079693 discloses a method of using a compound introduced with 3-oxo-5-phenyl-1,4-pentadienyl group as a side chain (skeleton of the 13th to 17th positions) at the position corresponding to the 12th position of latanoprost as a preparation intermediate.
Further, although not methods for preparing latanoprost per se, methods for preparing compounds that can be used as a precursor thereof are also known.
For example, a method of preparing 13,14-dihydro-17-phenyl-18,19,20-trinor-PGF2α from the Corey lactone is disclosed in Japanese Patent Unexamined Publication (Kokai) No. 48-18259, Example 3, No. 1, and a method of preparing 11,15-bistetrahydropyranyloxy-13,14-dihydro-17-phenyl-18,19,20-trinor-PGF2α from the Corey lactone is disclosed in Examples 3 to 7 and Example 25 of U.S. Pat. No. 3,971,826.
These preparation methods have problems that they are comprised of many steps although they have minor differences, and that target substance cannot be obtained in a total yield that is industrially satisfactory. In particular, when a reducing agent that does not have asymmetric distinguishability is used in the step of introducing hydroxyl group in a specific configuration at the position corresponding to the 15th position of latanoprost, a problem of heavy burden arises in later separation steps. They also have problems of using an expensive reagent such as (−)-chlorodiisopinocamphenylborane for asymmetric reduction, or a large amount of a reagent required for a substrate. Furthermore, they also have problems of using a dangerous raw material such as diisobutylaluminum hydride or readily occurred side reactions mainly at unprotected positions.    Non-patent document 1: J. Am. Chem. Soc., 91(20), pp. 5675-5677, 1969    Patent document 1: International Publication WO90/02553    Patent document 2: International Publication WO92/02496    Patent document 3: International Publication WO93/00329    Patent document 4: International Publication WO03/08368    Patent document 5: International Publication WO01/55101    Patent document 6: U.S. Patent Application Publication No. 2006/0079693    Patent document 7: Japanese Patent Unexamined Publication No. 48-18259